6.electrical-system. Electrical System
The electrical system in most light aircraft is a relatively simple installation that supplies power for engine starting, ignition (in some installations), avionics, lighting, fuel pumps, pitot heat, flap motors, landing gear (in retractable airplanes), and a variety of cockpit instruments and accessories. Although designs vary by manufacturer, nearly all modern reciprocating-engine airplanes use a 14-volt (single-battery) or 28-volt (twin-engine or larger) direct-current (DC) system powered by an engine-driven alternator and a storage battery.
Primary Components
- Battery — A lead-acid (or sealed AGM) battery, typically 12 V or 24 V, supplies power for engine start and serves as a backup source if the alternator fails. Battery output is also used to close the master solenoid that connects the battery to the bus.
- Alternator (or generator) — Engine-driven via a belt or gear, the alternator produces AC current that is rectified internally to DC. Alternators are preferred over generators because they provide rated output at lower engine RPM, are lighter for a given output, and maintain charging during taxi.
- Master switch — Usually a split rocker labeled BAT and ALT. Turning on BAT energizes the master solenoid and connects the battery to the main bus. Turning on ALT closes the alternator field circuit so the alternator can excite and produce output. The ALT half can be turned off independently to isolate a malfunctioning alternator without losing battery power.
- Voltage regulator — Maintains alternator output at a constant level (typically 14 V in a 12-V system) regardless of engine RPM or electrical load. It also controls the rate at which the battery is charged.
- Bus bar — A common terminal that distributes current from the source (alternator or battery) to individual circuits through circuit breakers or fuses. Larger aircraft may have multiple buses (main, avionics, essential) that can be isolated.
- Ammeter or loadmeter — The ammeter shows the rate of charge or discharge of the battery: a positive (right) deflection indicates the alternator is recharging the battery; a negative (left) deflection indicates the alternator is offline and the battery is supplying the load. A loadmeter instead displays the total current being drawn from the alternator, expressed as a percentage or in amperes; zero indicates an alternator failure.
- Circuit protection — Fuses, circuit breakers, or current limiters protect wiring from overload. A breaker that pops should be allowed to cool, then reset only once; if it trips again, leave it out and treat the affected circuit as inoperative.
- External power receptacle (optional) — Allows a ground power unit (GPU) to be plugged in for engine start or maintenance without depleting the battery.
Operation
With the engine off and the BAT master ON, all electrical loads are supplied by the battery and the ammeter shows a discharge. After engine start, the alternator comes online, the voltage regulator brings bus voltage up to roughly 14 V (or 28 V), the alternator carries the entire electrical load, and excess current recharges the battery. Once the battery is fully recharged, ammeter indication settles near zero (or the loadmeter shows only the actual system load).
During start, the battery delivers a very high momentary current to the starter motor, often 150-300 amps. This is why a weak battery may turn the prop slowly even though panel lights appear normal. After start, the pilot should verify a positive ammeter indication or appropriate loadmeter reading to confirm the alternator is online before taxi.
Alternator Failure
If the alternator fails in flight, the low-voltage warning light illuminates and the ammeter shows a discharge (or the loadmeter reads zero). The pilot should:
- Verify the failure by checking voltage and the ALT field circuit breaker.
- Cycle the ALT half of the master switch OFF then ON to attempt a reset (per the POH).
- If the alternator does not come back online, reduce electrical load to essential items only — typically COM 1, one NAV, transponder, and minimum lighting — and land as soon as practical.
A fully charged battery in a typical light single will support a reduced electrical load for roughly 30 minutes, though actual endurance varies with battery condition and load. Pitot heat, landing lights, strobes, and electric trim are heavy consumers and should be shed first.
Avionics Master and Bus Architecture
Many aircraft incorporate an avionics master switch that isolates radios and navigation equipment from voltage transients during engine start and shutdown. Always turn the avionics master OFF before starting and after shutdown to protect solid-state components.
In larger or more complex aircraft, the bus is split into a main bus and an essential (or emergency) bus. Some installations also include a standby alternator or a backup battery feeding the essential bus to power critical instruments — particularly important in glass-cockpit aircraft where the PFD, ADC, and AHRS depend entirely on electrical power.